Through air drying has become the technology of preference for making one-ply absorbent paper for many manufacturers who build new absorbent paper machines as, on balance, through air drying ("TAD") offers many economic benefits as compared to the older technique of conventional wet-pressing ("CWP"). With through air drying, it is possible to produce a single-ply absorbent paper in the form of a tissue with good initial softness and bulk as it leaves the absorbent paper machine.
In the older wet pressing method, to produce a premium quality printed, absorbent paper, it has normally been preferred to combine two plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, producing two-ply products, even on state of the art CWP machines, lowers paper machine productivity by about 20% as compared to a one-ply product. In addition, there may be a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Also, it is not normally economic to convert older CWP tissue machines to TAD. But even though through air drying has often been preferred for new machines, conventional wet pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air.
What has been needed in the art is a method of making a premium quality printed single-ply absorbent paper using conventional wet pressing having a high bulk and excellent softness attributes. In this way advantages of each technology could be combined so older CWP machines can be used to produce high quality printed single ply absorbent paper products in the form of bathroom tissue, facial tissue, and napkin at a cost which is far lower than that associated with producing two-ply absorbent paper. Two-ply absorbent papers are normally printed on the top ply. Any ink migration through the top ply (strikethrough) is hidden by the bottom ply, which also provides a barrier to further ink migration. In printing single-ply absorbent papers, it is important to prevent or minimize ink strikethrough onto process equipment, which can compromise process efficiency.
Among the more significant barriers to the production of printed single-ply CWP absorbent paper have been the generally low softness, thinness and the extreme sidedness of single-ply webs and their inability to hold the ink without having undesirable ink migration which renders the prior art one-ply products unprintable. An absorbent product's softness can be increased by lowering its strength, as it is known that softness and strength are inversely related. However, a product having very low strength will present difficulties in manufacturing and will be rejected by consumers as it will not hold up in use. Use of premium, low coarseness fibers, such as eucalyptus, and stratification of the furnish so that the premium softness fibers are on the outer layers of the tissue is another way of addressing the low softness of CWP products; however this solution is expensive to apply, both in terms of equipment and ongoing fiber costs. In any case, neither of these schemes addresses the problem of thinness of the web and the resulting unprintability of the absorbent paper product. TAD processes employing fiber stratification can produce a nice, soft, bulky sheet having adequate strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable. Because of the deficiencies mentioned above, many single-ply CWP products currently found in the marketplace are typically low end products which cannot be printed. These products often are considered deficient in thickness, softness, and exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in "away from home" applications in which the person buying the tissue is not the user. It should be not that to date there are no commercially printed one-ply CWP absorbent paper products.
We have found that we can produce a soft, printed, high basis weight, high strength CWP bathroom tissue, facial tissue, and napkins with low sidedness having a serpentine configuration by judicious combination of several techniques as described herein. Basically, these techniques fall into five categories: (i) providing a web having a basis weight of at least 12.5 pounds for each 3000 square foot ream; (ii) optionally adding to the web a controlled amount of a temporary wet strength agent and softener/debonder; (iii) low angle, high percent crepe, high adhesion creping giving the product low stiffness and a high stretch; (iv) optionally embossing the tissue; and (v) printing one or both sides of the absorbent paper product either before or after embossing. By various combinations of these techniques as described, taught, and exemplified herein, it is possible to almost "dial in" for the printed absorbent paper the required degree of softness, strength, and sidedness depending upon the desired goals. The use of softeners having a melting range of about 1.degree.-40.degree. C. and being dispersible at a temperature of about 1.degree.-100.degree. C. suitably 1.degree.-40.degree. C. preferably 20.degree.-25.degree. C. further improves the properties of the novel printed, one-ply absorbent paper product having a serpentine configuration.
The confirmation that our products have a very low printed sidedness was obtained by printing the Yankee side and the air side of the absorbent paper and comparing the differences. Surprisingly, on visual inspection, no differences could be ascertained and by the use of a spectrodensitometer, the total color difference (.DELTA.E) values supported the visual observation.
Samples were measured with an X-Rite 938 spectrodensitometer. A solid tone was measured for L*C*H.degree. color space coordinates and .DELTA.Ecmc using a 4 mm aperture, D65 light source, 100 standard observer, 2:1:1 factor setting. As described in the X-Rite Color Guide and Glossary, L*C*H.degree. is a three-dimensional cylindrical representation of color, where L* depicts Lightness, C* depicts Chroma (saturation), and H.degree. depicts Hue angle. The X-Rite 938 Operation Manual defines .DELTA.Ecmc as a single numeric value that expresses total color difference between a sample and a standard. CMC tolerancing is a modification of the L*C*H.degree., providing better agreement between visual assessment and instrumentally measured color difference. The CMC calculation mathematically defines an ellipsoid around the standard color with semi-axis corresponding to hue, chroma, and lightness and allows for a user defined acceptance level. An average of three measurements were reported. Differences in total color (.DELTA.E) were used to quantify similarity or differences in print appearance between the samples as a logical means to express relationships between the three-dimensional space of lightness, chroma, and hue angle. At an .DELTA.Ecmc value of .ltoreq.1.0, the standard observer would not detect differences in appearance between samples and at .DELTA.E.ltoreq.2.0, the differences would be very low. At .DELTA.E.gtoreq.3.0 differences would be readily observable. The backing ply was also measured for ink transfer using the same X-Rite settings. The amount of ink strikethrough on the backing ply was compared to white, non-print areas. Larger .DELTA.E levels indicate a greater total level of strikethrough. Relative differences between samples of .DELTA.Ecmc.ltoreq.1.0 indicate similar levels of strikethrough.